Traffic Surveillance and Traffic Management
Surveillance of air traffic is today managed by air traffic control (ATC) systems using primary and secondary radar. ATC systems currently under development use other or complementary techniques in the surveillance of air traffic. One such system is called automatic dependent surveillance-broadcast (ADS-B) which, on a long time scale, is expected to gradually replace current systems as a source for ATC information.
The basic idea of the ADS-B system is that all aircraft broadcast their own state vector, comprising position and status information, to all nearby aircraft and ground stations. Thus each aircraft has a complete picture of the surrounding traffic and the traffic close to a ground station can be monitored on ground.
Central to the ADS-B concept is the airborne data link (transponder) enabling the intended functionality. There are currently three different types of data links under consideration; Mode S ES, VDL Mode 4 and UAT.
Mode S ES is an extension of the conventional Mode S secondary surveillance radar system. VDL Mode 4 is a newly developed standard for a data link transponder compatible with ADS-B requirements. UAT is only considered for general aviation in the US.
Collision Avoidance and Separation Provision
Today, collision avoidance and separation provision is mechanized by air traffic controllers, pilots or the TCAS system. The basic feature in the TCAS system is the use of transponders and antennas. A very simplified explanation of the TCAS system is that it sends out a request from the transponder. If another aircraft is in the vicinity, an answer is sent back to the TCAS system which then knows the distance to the other aircraft (by measuring the time from the request to the received answer) and typically also in what direction the other aircraft is located (by using a directional antenna). The TCAS system then uses this information to issue warnings and suggested resolution manoeuvres if found necessary.
A drawback of today's TCAS systems is the uncertainties in the determined relative position (especially at large distances) due to the uncertainties in the directional antennas and the estimated distances to surrounding aircraft. These uncertainties may result in nuisance warnings from the system. The TCAS systems of today are therefore not considered to be suitable candidates for future collision avoidance and/or separation provision systems.
The ADS-B system and its possibility to automatically provide each aircraft with information relating to the surrounding traffic opens up for functionality such as automatic or semiautomatic separation provision and collision avoidance. These functions are particularly important in flight control of unmanned aerial vehicles (UAVs) but may also be important as a precautionary feature in piloted aircraft.
Considering the known shortcomings of TCAS, ADS-B seems a well suitable candidate for future systems for collision avoidance and separation provision using combinations of sensors such as, e.g., cameras, radar and ADS-B transponders. Aircraft-based aircraft surveillance systems for collision avoidance and separation provision are sometimes called Sense & Avoid systems.
Unfortunately, ADS-B systems of today suffer from a drawback. The position information received from surrounding air traffic has to be trusted to be correct. This is both a safety and security problem, safety in the sense that if the transmitting system emits an erroneous position it might cause a hazardous situation, and security in the sense that the system becomes prone to malicious use by emitting faked position reports.
For example, if an ADS-B message indicates an erroneous position of the aircraft from which it is transmitted, decisions made on the basis of that ADS-B message may have devastating consequences. An operator of an ATC system based on ADS-B data or a pilot/autopilot of an aircraft utilizing an ADS-B-based aircraft surveillance system, may be fooled to order/control an aircraft towards instead of away from the aircraft transmitting the erroneous ADS-B message.